Plasma enhanced chemical vapor deposition (PECVD) a-SiCXNY:H thin films have been studied as molecular sieve membranes for light (hydrogen, helium) gas separation at room temperature up to 150 degrees C. The microstructure of the deposited materials and their thermal stability have been characterized as a function of the electric plasma power and the ratio between the vapor precursors - hexamethyldisilazane and ammonia - using SEM, Fourier transformed infrared spectroscopy (FTIR), EDS, thermo-gravimetric analysis (TGA) and ellipsometry. Single gas permeation tests have also been carried out. An ideal selectivity of helium over nitrogen of 50 has been obtained at 150 degrees C with a helium permeance of about 10(-7) mol m(-2) s(-1) Pa-1. These promising PECVD materials developed for gas separation also revealed a chemical stability up to 500 degrees C, even in oxidative atmosphere. (c) 2008 Elsevier B.V. All rights reserved